The hard drive does not need to be reformatted; the OS has to be told to load the AHCI drivers early in the boot process. Win7 made this remarkably easy; I think it can be done by changing the state of two services. I'll race you to a forum search for the info.
Here's one: http://tweaks.com/windows/44119/improve-sata-hard-disk-performance-convert-from-ide-to-ahci/
Here's one on Tom's: http://www.tomshardware.com/forum/254925-32-ahci-option-sata
And a gruesome one: Change the BIOS to AHCI, boot from the Win7 DVD, and do a repair installation. Seriously, I don't recommend this except as a second-to-last try. The last try is a new install with the controller set to AHCI mode.